Tuesday, January 26, 2010

Man's best friend is much more than a household companion - for a few centuries, artificial selection in dogs has made them prime examples of the possibilities of evolution. A century and a half ago, Charles Darwin recognized how the incredibly diverse dogs supported his revolutionary theory in his infamous "On The Origin Of Species." At the time, he believed that dogs varied so much that they must have been domesticated from multiple canine species. Even still, he speculated that:

if... it could be shown that the greyhound, bloodhound, terrier, spaniel and bull-dog, which we all know propagate their kind truly, were the offspring of any single species, then such facts would have great weight in making us doubt about the immutability of the many closely allied natural species...inhabiting the different quarters of the world.

If only Darwin knew what we know now, that indeed, all dogs did descend from one species! While humans have been breeding dogs for over ten thousand years, it was until recently that strict breeds and the emphasis on "purebreds" has led to over 400 different breeds that are some of the best examples of the power of selection. Those that doubt that small variations in traits can lead to large levels of diversity clearly haven't compared a Pug to a Great Dane - I mean, just look at them compared to their ancestor:

We've turned a fine-tuned hunting animal, the wolf, into a wide variety of creatures, from the wolf-looking shepherds to the bizarre toy breeds. Before domestication, dog's life was tough, but when people pulled specific wolves out of their packs and began breeding them, we changed everything. There were some traits that made this easy - the social structure of wolves, for example, made them predisposed to belonging to a community. But we opened up a number of genetic traits and allowed them to express variety that would have been fatal in the wild. We not only allowed these traits to persist, we encouraged them. We picked dogs that were less aggressive or looked unique. And in doing so, we spurred on rapid diversification and evolution in an unbelievable way.

Take their skulls, for example. Like other members of the order Carnivora, dog's skulls have a few distinctive characteristics: relatively large brains and a larger-than-normal structure called a zygomatic arch which allows for bite power and chewing. But years of hand-picked puppies has led to an amazing amount of skull diversity in dogs. A study recently compared the positions of 50 recognizable points on the skulls of dogs and compared them to each other and other members of the order Carnivora. There was as much variety in the shape of the skulls of dogs as in the entire rest of the order, and the extremes were further apart. What does that mean, exactly? It means that the differences between the skulls of that Pug and Great Dane I mentioned before (on R) are greater than the differences between the skulls of a weasel and a walrus. Much of the variation in dogs is outside the range of the rest of the order, meaning their shapes are entirely unique. In just a couple centuries, our choices have created unbelievable variety in the heads of dogs - more than 60 million years has created in the rest of the carnivores.

The amazing diversity of dogs is a testimonial to the possibilities of selection. And it's not just their skulls that vary. A joint venture between the University of Washington and the Veterinary School at UC Davis mapped the variation in the genomes of 10 different breeds of dogs. They found that at least 155 different regions of the dog's genome show evidence of strong artificial selection. Each region contained, on average, 11 genes, so it's harder to identify exactly what about each area was under the most selection, though there were clues. About 2/3 of these areas contain genes that were uniquely modified in only one or two breeds, suggesting they contain genes that are highly breed-restricted like the skin wrinkling in the Shar-Pei. Another 16 had variations in 5 or more breeds, suggesting they encode for traits that are altered in every breed, like coat and size.

While we usually think of evolution as a slow and gradual process, dogs reveal that incredible amounts of diversity can arise very quickly, especially when selective pressures are very, very strong. It's not hard to see how selection could lead to the differentiation of species - just look at the over 400 breeds of dogs that exist today. There's a reason that you don't see many Chihuahua/Saint Bernard mixes: while it's entirely possible for their genetics to mix, it's just physically difficult for these two breeds to actually do it. Just imagine what a poor Chihuahua female would have to endure to give birth to such a mix, or how hard it would be for male Chihuahua to mount a female Saint Bernard. Indeed, dogs are well on their way to speciation.

Of course, it’s at this point that I have to mention that while I have talked about “dogs” this entire time, they’re not actually a different species. Wolves are Canis lupus, while dogs are merely a subspecies of wolves, Canis lupus familiaris. Despite centuries of selective breeding and the vast array of physical differences, dogs are still able to breed with their ancestors.

When you take away the selective breeding done by humans, a number of these unique traits disappear. But feral dogs don't just become wolves again - their behaviors and even looks depend greatly on the ecological pressures that surround them. Our centuries of selective breeding have opened a wide variety of traits, both physical and behavioral, that may help a stray dog survive and breed.

A good example of what happens to dogs when people are taken out of the picture lies in Russia’s capital city. Feral dogs have been running around Moscow for at least 150 years. These aren't just lost pets that band together – these dogs been on their own for awhile, and indeed, any poor, abandoned domesticated canine will meet an unfortunate fate at the hands of these territorial streetwalkers. Moscow's dogs have lost traits like spotted coloration, wagging tails and friendliness that distinguish domesticated dogs from wolves – but they haven’t become them. The struggle to survive is tough for a stray, and only an estimated 3% ever breed. This strong selective pressure has led them to evolve into four distinct behavioral types, according to biologist Andrei Poyarkov who has studied the dogs for the past 30 years. There are guard dogs, who follow around security personnel, treating them as the alpha leaders of their packs. Others, called scavengers, have evolved completely different behaviors, preferring to roam the city for garbage instead of interacting with people. The most wolf-like dogs are referred to as wild dogs, and they hunt whatever they can find including cats and mice.

But the last group of Moscow's dogs is by far the most amazing. They are the beggars, for obvious reasons. In these packs, the alpha isn't the best hunter or strongest, it's the smartest. The most impressive beggars, however, get their own title: 'metro dogs'. They rely on scraps of food from the daily commuters who travel the public transportation system. To do so, the dogs have learned to navigate the subway. They know stops by name, and integrate a number of specific stations into their territories.

This dramatic shift from the survival of the fittest to the survival of the smartest has changed how Moscow's dogs interact with humans and with each other. Beggars are rarely hit by cars, as they have learned to cross the streets when people do. They've even been seen waiting for a green light when no pedestrians are crossing, suggesting that they have actually learned to recognize the green walking man image of the crosswalk signal. Also, there are fewer "pack wars" that once were commonplace between Moscow's stray canines, some of which used to last for months. However, they remain vigilant against the wild dogs and wolves that live on the outskirts of the city – rarely, if ever, are they permitted into Moscow. When politicians thought to remove the dogs, their use as a buffer against these animals was cited as a strong reason not to disturb them.

Moscow's exemplary dogs show how different traits help dogs adapt to different ecological niches – whether it be brute strength for hunting in the truly feral wild dogs or intelligence in the almost-domesticated beggars. Some wonder if the strong selection for intellect will make Moscow’s metro dogs into another species all together, if left to their own devices.

Dogs make it easy to understand and demonstrate the core principles of evolution – variation and selection – and how they can make such a dramatic impact on an animal. It's no wonder that Darwin took cues from domesticated animals when formulating his theory of evolution. However, there's still a lot to learn about the processes that have shaped our best friends, and what future lies for them. How much time will it take to completely separate dogs from wolves, into their own species? What areas of the genome are key to doing so? In studying dogs and wolves, we may gain insight into how speciation occurs and when a threshold of change is met for it to do so. Seeing how much change has occurred already makes you wonder what surprises our canine companions still have in store for us as they, and we, continue to evolve together over the next ten thousand years.

There's a reason that you don't see many Chihuahua/Saint Bernard mixes: while it's entirely possible for their genetics to mix, it's just physically difficult for these two breeds to actually do it. Just imagine what a poor Chihuahua female would have to endure to give birth to such a mix, or how hard it would be for male Chihuahua to mount a female Saint Bernard. Indeed, dogs are well on their way to speciation.

Is it really known for certain that all dog breeds have compatible genomes?

Of course, it’s at this point that I have to mention that while I have talked about “dogs” this entire time, they’re not actually a different species. Wolves are Canis lupus, while dogs are merely a subspecies of wolves, Canis lupus familiaris. Despite centuries of selective breeding and the vast array of physical differences, dogs are still able to breed with their ancestors.

Given the morphological diversity among dogs, perhaps a different definition of species would be appropriate in this case. Lots of other 'species' can interbreed and have fertile offspring.

That is a good point, Bjørn. I guess it really depends on how we define a species. I don't think it's been tested, but I'm confident that if you took sperm from a Chihuahua and put it in a female Saint Bernard that the puppies would be just fine. But does that mean they're the same species?

As we define them now, species aren't solely based on breeding, though if it can't occur, it is a clear indicator of distinct species. Maybe we should look more for natural tendencies - as in, if you put a horny male of X in a room with Y in heat, do they mate? Do they recognize each other as the same species? But even that has it's issues and problems as a working definition.

Further aside, what's the point of defining species in a domesticated construct? In the wild we might talk about populations, conservation, etc, but when we're talking about dogs that are reared and bred by people, what is the meaning or importance of defining species?

I know that if you inseminate a Shetland pony mare with a Clydesdale stallion's sperm, she drops a foal that's Shetland sized when it's born. But it grows much larger than any pure Shetland pony, if not as large as a Shetland. (This has been done.) Probably the same thing would happen with your Chihuahua/St Bernard mix.

Anecdotally, my sister has a dog that looks like a giant Aussie shepherd - its father was actually a Great Pyrenees, but nothing of his came over into this F1 litter than his size. Of course, Pyrenees and Aussies are much closer in size to each other than Chihuahuas and St Bernards!

I've been told that when a tiny female dog mates with a large male, the delivery isn't inordinately problematic, not nearly as you might think. "Nature just takes care of itself." Call the dog expert warehouse and ask them to send one over!

Excellent piece on dogs [I'm a fan of, and we breed, Portugese Water Dogs] Recently an article in SCIENCE examined the genetics of certain coat variations in dogs, concluding that quite a few variations were due to differences in only 3 genes. But the study omitted the most significant difference, between non-shedding [hair] coats and shedding [fur] coats. Neither authors nor Journal responded to the question of cause of this difference. Do you know, or know of, the genetics of this difference? non-shedders include our PWDs, Poodles, some others.Richard Frankel [PhD, Physical Biochem.]

Came back from your blog about the Research Blog Awards to read this, and I agree its a worthy candidate for Research Blog of the Year.

I just wondered about the last sentence of your "Take their skulls" paragraph. You contrast "a couple of centuries" of our choices with 60 million years [evolution] for the rest of the carnivores. If human beings have been around for a million or two of those sixty million years, what evidence is there for when men began domesticating dogs? Surely it would be more than a couple of centuries ago, though if you count from the origins of the Westminster Dog Show you might be closer.

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Why Do I Blog?

I started this blog mostly because of my friend Allie. She's on her way to a masters in Journalism, and she got assigned to write a blog for one of her classes. When she told me about it, I thought"wow, good idea."

You see, I love to write, but as a scientist very little of my day-to-day writing is interesting or legible to anyone who isn't a scientist. So this lets me write in a way that is actually fun to read - I know, novel.

Anyhow, if you want to why it's called "Observations of a Nerd," read this.